According to their model (Henry Tye and Gia Dvali), during the period of inflation, two branes were moving towards each other, and as they got closer and closer, the interaction between the branes drove the inflation. The branes are attracted to each other and collide because one is a brane and one is an anti-brane. These branes would eventually collide. Henry Tye: "And when they collide, they annihilate. And when they annihilate, they release a lot of energy." According to Tye's theory, this collision marks the end of inflation. And the vast amount of energy released in the collision gets converted to matter. It is the moment in the Big Bang when all the matter in the universe as we know it is created. And since strings are the fundamental building blocks in string theory, this matter is in the form of strings. These strings are created in various sizes, some large enough to stretch across our entire universe. These are called "cosmic strings."